1. Field of the Invention
This invention relates to a method and an apparatus for preventing erroneous operation in a non-contact push-button switch which is used as a pendant switch of hoist crane, for example.
2. Description of the Prior Art
Generally, as the pendant switch mentioned above, a contact type push-button switch has been used. However, in operating the push-button switch of this type, sparking takes place upon each operation resulting in an erosion of the contact and the service life of the switch is shortened. Also, in a low-voltage operation, an inferior contact takes place and the crane operation thus lacks accuracy. Moreover, in the case of a contact type multi-stage operation switch, the switch assembly must be made large.
In view of the above drawbacks of the contact type push-button switch, the present inventors previously suggested a non-contact type push-button switch which utilizes light (Japanese Patent Application No. 58-77860 and Japanese Patent Application No. 60-94294). This switch comprises a luminous element, a light receiving element and a push-button having a light shading plate provided between both elements and is designed s that light projection to the light receiving element is selectively controlled according to the position of the push-button.
Since the above pendant switch is suspended from a hoist crane, it is more readily subject to impact, vibration, etc.. That is, it must operate under severe conditions. Electronic parts of the push-button switch will more readily break-down than electronic parts contained in a control box, for example, when the switch breaks, if an operation signal is not given in spite of the operation of the push-button, although the crane is inoperable, the danger presented is minimal. However, if an operation signal is given continuously in spite of restoring the push-button to an OFF position, unintended movement of the crane will result, thus creating a very dangerous situation.